A number of applications involve the spraying of coating material onto a surface which is hidden from view. For example, it is common practice among vehicle manufacturers to apply wax or other protective coatings to the inner cavities of vehicle body components such as doors, rear decks and hatchback lids, hoods, fender panels and under bodies. The hem flange areas of such components, and other areas where water and corrosive materials might collect, are often hidden from view and a spray device such as a spray gun must be manipulated in a manner to obtain complete coverage of the area to be coated.
In order to ensure application of a protective coating to the desired locations on such vehicle body components, and with the desired uniformity, spray coating systems have been proposed such as disclosed in U.S. Pat. No. 4,613,528 to Mueller, which is assigned to the same assignee as this invention. The system disclosed in the U.S. Pat. No. 4,613,528 is a mechanical system in which one or more spray guns are placed in the proper position with respect to a vehicle body component to be coated and then automatically manipulated to obtain complete and uniform coverage of the vehicle body component.
One problem with mechanical systems of the type described above, or manual systems for coating vehicle body components, is that no means are provided to indicate to the operator whether or not the spray guns used to apply the coating material are functioning properly. In the course of a spraying operation wherein the spray guns of such systems are completely hidden from view, the operator has no way of determining whether or not the spray guns are discharging coating material at the desired flow rate. Clogging of such spray guns can be a recurring problem when applying wax and similar viscous coating materials which must be maintained at certain temperatures in order to flow freely through the spray guns. Such temperatures might not be maintained throughout a production run, particularly when the spray guns are operated intermittently and/or where the system does not provide for recirculation of the coating material from the spray gun back to the source of coating material when the spray guns are not being operated.